The important step in the manufacture of semiconductor chips and thin film circuitry is the etching of the different layers such as polysilicon and silicon which make up the finished semiconductor chip or the thin film circuit. In the manufacture of thin film circuits, one method of etching has been to overlay the surface to be etched with a suitable mask and immerse the circuit so masked in a chemical solution which attacks the surface to be etched while leaving the mask intact. It has been difficult with the chemical etching processes presently known to achieve well defined edges on the etched surfaces. The difficulty arises because the chemicals used for etching tend to undercut the mask, i.e., the rough chemical seeps under the mask and continues to attack the surface to be etched even under the mask area. A related difficulty which is encountered with certain materials is that the chemical action tends to eat through the surface to be etched and attacks the substrate beneath. It is, therefore, very difficult to use wet chemical etching to achieve fine structures. Fine structures being defined as structures having geometries on the order of one micron.
Etching of thin film circuits has also been done by a process sometimes called sputter etching. Typically a container such as a bell jar is filled with an inert gas such as argon. In the container are placed an anode and a cathode, the latter of which is negatively biased relative to the former, e.g., by means of an applied rf signal. A surface to be etched is covered by a suitable mask of a material such as photoresist, silicon nitride, etc. and is then placed on the cathode. When a negative bias is applied to the cathode the inert gas in the region between the cathode and the anode is ionized and the positive ions are attracted towards the cathode. Those ions which strike the surface to be etched serve to knock atoms off the surface thereby gradually etching through the material. In this process the photoresist material forming the mask is also etched typically at about the same rate or faster rate than the surface to be etched. Therefore thick masking layers on which it is difficult to hold tolerances are required. Although this sputter etching process produces better defined edges than the chemical etching process, it is typically very slow, especially on tantalum, or nitride surfaces which are important in thin film work.
In the manufacture of semiconductor chips, another procedure (sometimes called plasma etching) is used in which a container such as a bell jar is filled with a gas such as CF.sub.4, or SF.sub.6 , whose constituent ions are chemically reactive. A surface to be etched is covered by a mask and inserted into the container along with the reactive gas. To etch the surface, an rf exciting coil around the container is activated to excite the CF.sub.4 or SF.sub.6, thereby disassociating the CF.sub.4 or SF.sub.6 and forming various positive and negative ions. The disassociated ions apparently then chemically interact with the surface to be etched producing various gases as a reaction product. As with the wet chemical etching process described above, this type of plasma etching also results in undercutting of the mask areas so that it is difficult to achieve well defined edges.
Reactive ion etching or plasma ion etching is well known in the prior art. The following prior art patents and summaries are submitted to generally represent the state of the prior art.
Reference is made to U.S. Pat. No. 3,573,192, entitled "Plasma Generating Apparatus" granted March 30, 1971 to R. L. Bersin et al.
The abstract of the disclosure of the Bersin et al patent reads as follows:
"A plasma, or ionized gas, generating apparatus for reaction of the plasma with non-gaseous substances in a container, the apparatus having a support structure defined by spaced apart support plates which are interconnected by a plurality of posts. A pair of elongate electrode plates for each container is mounted on the support structure so that at least one of the plates can be moved toward and into engagement with the container for grasping the container and mounting it on the apparatus. The electrode plates have a concave configuration to grasp the container and include a layer of an electrically insulating material on the concave side to prevent short circuits between the electrode plates and the container and to facilitate the slideable insertion and removal of the container from between the plates. A supply manifold is carried by the support structure and includes a plurality of supply conduits for connection with the containers and the transmission of gas into the containers. An exhaust manifold is similarly carried by the support structure and connected with the container for removal of the gas and the plasma from the container."
Reference is made to U.S. Pat. No. 3,679,502 entitled "Gaseous Nonpreferential Etching of Silicon", granted July 25, 1972 to R. G. Hays. The Hays patent discloses a silicon surface etched or polished with a gaseous mixture comprising sulfur hexafluoride SF.sub.6 of high purity and a carrier gas such as hydrogen at temperatures between 950.degree. C. and 1250.degree. C. The sulfur hexafluoride should have a low nitrogen concentration with a preferred nitrogen concentration being less than 200 parts per million by weight.
Reference is made to U.S. Pat. No. 3,806,365 entitled "Process for Use in the Manufacture of Semiconductive Devices", granted Apr. 23, 1974 to A. Jacob. The ABSTRACT of the Jacob patent reads as follows:
"A process step for use in the manufacture of semiconductor devices. To enable the removal of all the photoresist material along with its inorganic contamination, after development and etching of preselected portions of an oxide layer on a semiconductor slice, the material is exposed to a low pressure (few torr) RF generated `cold` plasma (200.degree.-300.degree. C.), where the plasma is a homogenous gaseous mixture of oxygen and organo-halides. The organo-halide preferably is a binary or ternary mixture where each component preferably includes no more than two carbon atoms per molecule and is desirably fully halogen-substituted. One of the substituents should include a predominance of either fluorine or fluorine-bromine combinations."
Reference is made to U.S. Pat. No. 3,880,684, entitled "Process for Preparing Semiconductor", granted April 29, 1975 to H. Abe. A semiconductor is prepared by continuously etching at least two types of silicon compound layers, such as silicon dioxide (SiO.sub.2), silicon nitride (Si.sub.3 N.sub.4) or polycrystalline silicon which are formed on a silicon substrate. A freon gas plasma is used for etching so that the two types of silicon compound layers are continuously etched in a sloped form with undercutting, as occurs in conventional chemical solution etching.
Reference is made to U.S. Pat. No. 3,923,568 entitlted "Dry Plasma Process for Etching Noble Metal" granted Dec. 2, 1975 to R. L. Bersin. The Bersin patent discloses a process for etching noble metals, particularly, for removing selected areas of thin films of electrically conductive noble metals, by contacting exposed areas of noble metal with a plasma that must include both fluorine and chlorine and may, optionally also contain oxygen.
Reference is made to U.S. Pat. No. 3,971,684, entitled "Etching Thin Film Circuits and Semiconductor Chips", granted July 27, 1976 to S. Y. Muto. The Muto patent discloses a method of etching either thin film circuits or semiconductor chips which is capable of producing extremely well-defined edges on etched materials, while at the same time achieving rapid etching rates. According to the method a gas or gas mixture whose constituent ions are chemically reactive is placed in a container along with a cathode electrode and an anode electrode. A surface to be etched is covered by a suitable mask and mounted on one of the electrodes, e.g., the cathode which is negative-biased relative to the remaining electrode, e.g., by applying an RF biasing signal. An electric field is thus established in the region between the cathode and the anode, and serves to dissociate the reactive gas. Chemically reactive gas ions are attracted to the cathode and thereby impinge on the sample to be etched. Apparently, the surface is etched both by chemical interaction with the active ions and by the momentum transfer of the ions impinging on the surface. By virtue of the electric field attracting ions to the cathode, the ions impinge on the surface to be etched predominantly in a direction perpendicular to that surface, so that the process produces well-defined vertically etched sidewalls. Chemically reactive gases such as SF.sub.6 or CCl.sub.2 F.sub.2 may be employed, however, CF.sub.4 is preferred.
Reference is made to U.S. Pat. No. 3,994,793 entitled "Reactive Ion Etching of Aluminum", granted Nov. 30, 1976 to J. M. Harvilchuck et al. The Harvilchuck et al patent discloses a process for reactive ion etching of aluminum wherein a masked layer of aluminum supported on a substrate is exposed to an RF plasma formed by imposing an RF voltage across at least two spaced electrodes in a gaseous environment composed of an inert gas and a gas selected from the group consisting of CCl.sub.4, Cl.sub.2, Br.sub.2, and HCl.
Reference is made to U.S. Pat. No. 4,026,742, entitled "Plasma Etching Process for Making A Microcircuit Device", granted May 31, 1977 to K. Fujino. The Fujino patent discloses a method of making a microcircuit device wherein a uniform film of electrically conductive metal is deposited on the microcircuit surface and selectively removed from areas exposed through a mask. The improvement comprises the steps of contacting the exposed metal with a reactive halogenated gas in plasma state to convert the metal to a metal halide; and removing the metal halide to form a pattern of electrically conductive metal on the device. The plasma can be generated in a reaction chamber with a high frequency electromagnetic field. The process is useful in forming a desired pattern of metal for electrodes or wiring on a semiconductor substrate or other microcircuit base. The process is particularly useful for etching tungsten or molybdenum metal patterns. The preferred halogenated materials include prehalogenated organic compounds, such as trichlorofluoromethane, dichlorodifluoromethane, or other volatile organic compounds containing halogen atoms having an atomic number between 9 and 35, especially chlorine and fluorine atoms; however, brominated compounds such as CHBr.sub.3, CH.sub.2 BR.sub.2 or CH.sub.3 BR, may also be used.
Reference is made to U.S. Pat. No. 4,052,251 entitled "Method of Etching Sapphire Utilizing Sulfur Hexafluoride" granted Oct. 4, 1977 to C. E. Weitzel. The Weitzel patent discloses a process for forming a blind hole having an isosceles trapezoidal cross-section in a sapphire substrate using a sulfur hexafluoride gas etchant and an etch mask of silicon nitride on top of silicon dioxide. A composite of sapphire, silicon dioxide and silicon nitride wherein silicon dioxide is located in between the sapphire and the silicon nitride are congruently apertured.
Reference is made to U.S. Pat. No. 4,069,096 entitled "Silicon Etching Process" granted Jan. 17, 1978 to A. R. Reinberg et al. The Reinberg et al patent discloses a process for etching silicon including the step of contacting said silicon with a plasma derived from a gas comprising CCl.sub.4, an inert gas, and a gas selected from the group consisting of Cl.sub.2 and HCl.
Reference is made to U.S. Pat. No. 4,094,732, entitled "Silicon Etching Process" granted June 13, 1978 to A. R. Reinberg. The Reinberg patent discloses a process for etching silicon including the step of contacting the silicon with a plasma derived from a gas comprising CCl.sub.4 and an inert gas. The inert gas is nitrogen or argon.